The present invention relates in general to externally threaded plastic closures for plastic drums of the type where the opening(s) in the drum have standard two inch (5.08 cm) NPS threads. More particularly, the present invention relates to plastic closures which have a gasket encapsulating structure in the closure flange and external threads arranged with a straight portion and a lower tapered portion.
Drum manufacturers and plastic drum reconditioners have the majority of their processing problems either manufacturing or servicing the two inch (5.08 cm) NPS threaded opening. Because of the inherent design of the NPS thread (originally designed for steel pipe fittings), drum manufacturers have a very difficult time manufacturing a drum which will perform to the required Department of Transportation and United Nations regulations. Typically the first area to fail is the two inch (5.08 cm) NPS threaded opening. Reconditioners have significant problems with the NPS opening ovalizing and shrinking to the point where a plastic drum closure cannot be screwed into the opening in the drum end. However, drum users want to maintain the NPS thread due to adaptability of existing pumps and faucets.
The present invention provides an improvement and solution to the foregoing problem by a unique thread configuration and gasket encapsulating structure. The present invention addresses the problems of ovalization and shrinkage at the reconditioners and performance problems at the drum manufacturers. In the present invention the external threads on the closure are configured with one portion which is straight, exhibiting a constant pitch diameter, while the bottom most portion (bottom three threads) is tapered, or has a reducing pitch diameter. The pitch of both the straight and tapered threads is constant and this feature allows the closure to be installed in drum openings which have ovalized or shrunk, problems which are typically experienced by drum reconditioners. This opening or outlet distortion is caused by the temperature of cleaning solutions involved in the reconditioning process. Distortion is promoted due to the relaxation of material stresses which are introduced in the original manufacturing process of the plastic drum and its opening.
The gasket encapsulating feature of the present invention improves the performance of a plastic drum with respect to maintaining a seal when pressurized. When a plastic drum is pressurized there is noticeable distortion and it may radically change shape. Typically this distortion causes the drum closures to leak because there is not constant pressure exerted on all areas of the sealing gasket. The present invention is designed to flex in cooperation with the distortion of the opening and the gasket encapsulating means disclosed by the present invention keeps the relative softer gasketing material in proper placement so as to effect a liquid-tight seal. Without the gasket encapsulating arrangement as part of the closure flange, a gasket would tend to deform outwardly when the container is exposed to higher internal pressures. Attempts to restrict this gasket movement using only the configuration of the drum outlet has proven to be ineffective. However, putting the gasket encapsulating feature of the present invention on the closure flange has solved the problem of gasket movement.
There is one additional benefit to the design of the present invention which is a result of the required manufacturing method. Due to the relationship of the undercut groove in the flange (gasket encapsulated feature) and the threads, the plastic closure of the present invention must be unscrewed from a mold cavity. Typically, external threads are molded using some arrangement of collets or half moon split blocks which are closed in the molding position and opened in the ejection process. With normal tooling wear this causes flash at the interfaces of the collet jaws or split block mating surfaces. Flash which is formed is likely to break off from the closure when the closure is applied to the drum outlet. Any flash which does break off will likely land in the drum, contaminating the contents of the drum with minute plastic particles. Since the threads on the present invention are unscrewed from the mold cavity, there are no mating tool surfaces to wear in a manner that permits flash and thus there is no flash in the threaded area nor in the container.
Over the years a number of closure designs have been invented to address specific problems and concerns. In many of these earlier designs some type of seal member is provided in order to assist in sealing the interface between the container outlet and the closure. In some of these earlier designs there are special threads or modified threads, again, intended to address or solve some specific problem. The following list of patent references is believed to be a representative sampling of these earlier closure and mating thread designs:
______________________________________ Patent No. Patentee Issue Date ______________________________________ 4,190,171 Kulle et al. 2-26-80 2,008,245 Curtis et al. 7-16-35 2,906,429 Marchyn 9-29-59 3,346,278 Yocum 10-10-67 3,027,042 Graves 3-27-62 2,025,852 Dillhoefer, Jr. 12-31-35 3,664,540 Witkin 5-23-72 3,589,550 Rossmann 6-29-71 3,487,442 Rossmann 12-30-69 3,405,837 Carpenter, Jr. 10-15-68 3,380,618 Phillips 4-30-68 3,891,118 Laurizio 6-24-75 2,339,255 Dodson 1-18-44 2,445,802 Robinson 7-27-48 1,324,896 Hettinger 12-16-19 4,078,696 Crisci 3-14-78 3,122,262 Hagmann et al. 2-25-64 4,005,799 Mannaerts 2-1-77 5,211,304 Stolzman 5-18-93 4,146,207 Rofe 3-27-79 2,769,566 Thompson 11-6-56 2,291,706 Frease 8-4-42 GB 2 227 736 A UK-Christy 8-8-90 352256 Switzerland 2-15-61 ______________________________________
While the array of concepts and features represented by the foregoing listed patent references covers a wide variety of structures, the specific features and combination of features of the present invention are not anticipated. Further, in view of the focused and specific nature of the list of references and the inability to compatibly combine various features, the present invention is such that it would not be obvious to one of ordinary skill in the art in view of the listed patent references.